The present invention relates generally to lasers, and more specifically the invention pertains to a system for self-injection locking a single laser for reducing the linewidth of the laser's output.
Injection-locking is a phase synchronization technique that is commonly performed with two lasers. Self-injection locking has been accomplished with a single laser but does not reduce the linewidth of the output as narrowly as injection with two lasers. The present invention uses self-injection locking of a single laser using a prepulse to establish a low power beam that is line narrowed before self-injection to generate a high power pulse. The result is a self-injection locking system which outputs a narrow linewidth laser.
Laser injection locking was first demonstrated by Stover and Steier who injected the beam of one very stable Fabry-Perot laser into the resonator of a second very stable Fabry-Perot laser. They measured the locking range as a function of drive level of the locking signal and found that the experimental results compared favorable with the classical theory. Each of the two lasers operated in as single frequency mode. The length of each laser cavity could be adjusted to tune the frequency by means of a mirror mounted on a piezoelectric transducer. By sweeping one laser, the injection-locked region (or the frequency range free from the beating which is the result of simultaneous operation at the self-oscillation and driven frequency) could be measured. The locking bandwidths which ranged from 0.1 to 1.0 MHz were found to be proportional to the square root of the injection power.
With the advent of laser telescopes and similar applications of high powered lasers has come the need for long coherence lengths required for non-linear optical effects. Non-linear optical efforts require coherence lengths on the order of centimeters. The coherence length of the laser can be increased by line narrowing. Current methods use injection locking to narrow the spectral output. This is accomplished with a reference oscillation (RO) whose beam is injected into a power oscillator (PO). This method allows 60 percent of the power to be delivered in a 50 MHz bandwidth for a 20 ns pulse. This reduction in bandwidth is called line narrowing. The optical conversion efficiency for 2nd harmonic generation improves as the square of the coherence length. This method requires two or more lasers.
A second method is self-injection locking which requires only one laser. However, it requires a fraction of the beam to be sent into a frequency discriminating section of the laser. The frequency discriminating component can be prisms, gratings, or etalons. The self injection locking reduces the linewidth better than a geometrical analysis predicts, but still not nearly as narrow as true injection locking.
The task of providing a self-injection locking system which yields spectrally narrow laser outputs from high powered lasers is alleviated, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,635,266 issued to Facklam; PA1 U.S. Pat. No. 4,227,159 issued to Barret et al; PA1 U.S. Pat. No. 3,999,146 issued to Lang et al; PA1 U.S. Pat. No. 4,264,870 issued to Avicola et al; PA1 U.S. Pat. No. 4,490,823 issued to Komine et al PA1 U.S. Pat. No. 4,686,485 issued to Goldberg et al; PA1 U.S. Pat. No. 4,689,794 issued to Brosnan; PA1 U.S. Pat. No. 4,726,011 issued to Ih et al; PA1 U.S. Pat. No. 4,755,016 issued to DeLoach et al; and PA1 U.S. Pat. No. 4,768,852 issued to Ih.
All of the above-cited patents disclose prior art laser injection systems which can be improved by the present invention.
The Facklam patent discloses a dispersion control system for a laser. Barrett et al disclose an injection locking system using two lasers and tuning etalons. The semi-conductor laser of Lang et al is used with several injection locking devices, including both a modulating laser element and an injection laser element.
Avicola et al teach the use of an automatic locking system for an injection locked laser. Komine et al disclose an injection locked unstable laser. Goldbert et al disclose the optical injection locking of laser diode arrays.
The Brosnan patent teaches the conventional injection locking technique using two lasers. The injection laser's output enters the rear reflector of a xenon chloride laser to slightly shift its gain profile towards longer wavelengths.
The Ih patent discloses modulators and injection locked lasers.
The following technical articles are also pertinent to the present invention, and are incorporated herein by reference:
"Laser Injection Locking" by C. Buczek et al, Proceedings of the IEEE, vol. 61, No. 10, October 1973;
"Injection-Locking Unstable Resonator Excimer lasers," IEEE Journal of Quantum Electronics, Vol. QE-19, No. 9, Sep 83;
"The Influence of the Beam Expanding Prism in a Dye Laser Resonator on the Linewidth and its Dependence on the Expansion Ration," Optics Communication, Vol. 40, No. 1 P. 9, 1 Dec 81;
"Spectrally Narrow Pulsed Dye Laser Without Beam Expander," Applied Optics, Vol. 21, No. 17, p. 3065, 1 Sep 82;
"High Efficiency Interferometric Tuning of Flashlamp Pumped Dye Lasers," Optics Communication, Vol 4, No. 2, Oct 71;
"Spectral-narrowing techniques for excimer laser oscillators," Canadian Journal of Physics, Vol. 63, p. 214-219, 1985; and
"Performance of a Novel Injection-locked Excimer Laser," Journal of Applied Physics, Vol. 56, No. 7, p. 2172, 10 Oct. 84.
All of the references cited above teach state-of-the-art laser injection locking techniques. While these references are instructive, a need remains to provide a system for self-injection locking a single laser for reducing linewidth of the laser's output. The present invention is intended to satisfy that need.